1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus for preventing excessive handovers in a GSM (Global System for Mobile communications) system.
2. Description of the Related Art
In general, the second-generation (2G) mobile communication system providing a voice-oriented service uses a GSM scheme, an IS-95 (Interim Standard 95) scheme, and the like. A GSM system was commercialized from Europe in 1992. The GSM system provides a communication service using a Time Division Multiple Access (TDMA) scheme.
When moving across a boundary region between base stations (BSs) in a mobile communication system, a mobile terminal must perform a handover (or a handoff) so as to maintain its ongoing call. Handover is a process for switching a current call channel of a mobile terminal to another call channel such that an ongoing call of the mobile terminal can be maintained without interruption when the mobile terminal moves from a current cell into another cell.
Because the GSM system provides a communication service using the TDMA scheme, a GSM mobile terminal can perform a handover even during high-speed movement of 80-100 Km/h or more. However, the GSM system performs a call handover by a hard handover scheme, not a soft handover scheme by which a Code Division Multiple Access (CDMA) system performs a seamless call handover.
That is, during a handover process, after being allocated a channel for a handover from a network, the GSM mobile terminal disconnects a current call channel before switching to a new call channel. This disconnection of the current call channel temporarily interrupts an ongoing voice call of the GSM mobile terminal during the handover process.
The handover process of the GSM mobile terminal will be described in detail below. First, the GSM mobile terminal in a call mode (i.e., a dedicated mode) transmits a measurement report message to the network at fixed periods, for example, at every 104 frames or 102 frames. The contents of the measurement report message are shown in Table 1 below.
TABLE 1IndexNameValue1Serving BCCH ARFCNFrequency2RxLev Full Average0-633RxLev Sub Average0-634RxQual Full Average0-7 5RxQual Sub Average0-7 6Number of Entries0-6 7BCCH ARFCN [1]~[6]Frequency8RxLev Average0-639BSIC<ncc, bcc>
As shown in Table 1, the contents of the measurement report message are classified into contents about a serving cell and contents about a neighboring cell. The measurement report message is used to report a frequency of the serving cell (Serving BCCH (Broadcast Control Channel) ARFCN (Absolute Radio Frequency Channel Numbers), a received (Rx) signal level, an Rx signal quality level, the number of neighboring cells, an Rx signal level of a frequency out of each BA list, and a Base Station Identity Code (BSIC). The number of neighboring cells are referred as Broadcast Control Channel Allocation (hereinafter ‘BA’) list number'.
Here, when a discontinuous transmission (DTX) scheme is used, if the RxLev Full Average and the RxQual Full Average are not used, the RxLev Sub Average and the RxQual Sub Average are used.
Upon receipt of a Handover_Command message from the network, the GSM mobile terminal performs a handover from the serving cell to the neighboring cell. Thereafter, the GSM mobile terminal transmits a Handover_Complete message to the network, thereby completing the handover process. At this point, a voice call of the GSM mobile terminal is interrupted during the handover process. For reconnection of the voice call, the GSM mobile terminal resumes a GSM traffic channel (TCH). Thereafter, the GSM mobile terminal transmits the measurement report message to the network at every 104 message frames (MF). Accordingly, the GSM mobile terminal can return to the dedicated mode.
However, because the conventional GSM mobile terminal transmits the measurement report message to the network directly after returning to the dedicated mode, a ping-pong handover may occur. The ping-pong handover is a phenomenon in which a handover occurs repeatedly when the strength of power frequently changes in an inter-cell boundary region. The ping-pong handover may cause an overload at a system controller of a mobile switching center. In addition, frequent handovers may occur in the inter-cell boundary region. These frequent handovers may cause a failure in a handover and thus generate a call-drop and a noise, which degrades the stability and voice quality performance of the GSM mobile terminal during a call operation.